One particular case of such a structure is a debondable structure, in which the separation interface is an interface along which bonding via molecular adhesion has been carried out.
The expression “bonding via molecular adhesion” denotes bonding via intimate contact of the surfaces of the two substrates, employing adhesion forces, mainly van der Waals' forces, and not using an adhesive layer.
Without wishing to be limiting, it may, however, be considered that a debondable structure may be used mainly in four different applications:
a) bonding of a mechanical stiffener: it may be desirable to bond a mechanical stiffener to a weak substrate or thin layer in order to prevent damaging or breaking it during certain fabrication steps, then to be able to remove this mechanical stiffener when its presence is no longer needed.
b) rectifying poor bonding: debonding makes it possible to debond two substrates that might not have been correctly bonded a first time, then to rebond them after cleaning, in order to improve the profitability of a fabrication process and to avoid, for example, scrapping poorly bonded substrates.
c) temporary protection: during certain steps of storing or transporting substrates, especially in plastic boxes, it may be useful to temporarily protect their surfaces, especially those intended to be used subsequently for the fabrication of electronic components, in order to avoid any risk of contamination. One simple solution consists in bonding two substrates so that the faces thereof to be protected are bonded, respectively, to one another, then in debonding these two substrates during the final use thereof.
d) double transfer of a layer: this consists in producing a reversible bonding interface between an active layer and a first support substrate (optionally made of an expensive material), then in transferring this active layer to a second final substrate, by debonding the reversible bonding interface.
However, applications may also be found in which it is desired to separate a structure formed of two assembled substrates, along an interface, which is not a bonding interface.
Such an interface may be, for example, an interface between a first material and a second material, which may have been joined to one another by an addition of the second material to the first, for example, via deposition, epitaxy, etc.
As a variant, such an interface may be, for example, a weak zone formed within a material and marked by the presence of bubbles, inclusions, etc.
Separation along an interface, which is not a bonding interface, may in particular find an application in the transfer of a layer from a first substrate to a second substrate.
The layer to be transferred may, thus, not have been formed by bonding to the first substrate but, for example, may have been formed by epitaxy or deposition on the substrate, or, alternatively, may be part of a thicker layer within which it has been delimited by a layer of bubbles that weakens the thick layer.
Irrespective of the envisaged applications, it is necessary to carry out this separation without damaging, scratching or contaminating the surface of the two substrates located on either side of the separation interface and without breaking these two substrates.
Depending on the various applications, these two “substrates to be separated” may be two layers of one and the same substrate or two different substrates.
Moreover, the larger the dimensions of the two substrates of the structure to be separated or the higher their bond energy, the more difficult the separation is to carry out, in particular without damage.
Furthermore, it is known from the research studies by Maszara regarding the measurement of the bonding energy between two substrates (see the article by W. P. Maszara, G. Goetz, A. Caviglia and J. B. McKitterick: J. Appl. Phys. 64, (1988), 4943) that it is possible to measure the bonding energy between two substrates, by introducing a thin blade between the two, at their bonding interface.
Maszara established the following relationship:
  L  =                    3        ⁢                  Et          3                ⁢                  d          2                            32        ⁢                                  ⁢        γ              4  
in which d represents the thickness of the blade inserted between the two bonded substrates, t represents the thickness of each of the two bonded substrates, E represents the Young's modulus along the debonding axis, γ represents the bonding energy and L represents the length of the crack between the two substrates at equilibrium.
The above formula starts from the hypothesis that the two substrates are of identical dimensions.
Owing to the aforementioned relationship, it is possible, by measuring L, to determine the bonding energy γ.
This definition of the “bonding” energy is based on the hypothesis that the energy needed to separate the two substrates, or rupture energy of the interface (which is the energy actually measured by the method using a blade) is equal to the bonding energy of the substrates.
In reality, during the separation of the substrates, a portion of the energy is dissipated not in the rupture of the interface itself but in other phenomena, such as deformations of the material(s) present at the interface.
In the remainder of the text, the rupture energy of an interface will, therefore, denote the energy to be provided in order to separate two substrates or layers along the interface.
Insofar as these substrates or layers to be separated are stiff enough to be separated with a blade, it is possible to separate them by parting them sufficiently from one another, at their bevelled edge, which has the effect of creating a separation wave.
This wave propagates from the point of the edge of the substrates where it is initiated, across the entire surface of these substrates, along the separation interface.
When the structure consisting of the two substrates contains only one separation interface, the insertion of a blade between the two substrates and the application, via the blade, of a parting force on the substrates will have the effect of separating the substrates along the interface.
However, situations are frequently encountered in which the structure comprises more than one separation interface.
FIG. 1 schematically illustrates such a situation, in which the structure S is formed from a first substrate S1 and from a second substrate S2 and comprises two separation interfaces I1, I2, respectively, having rupture energies γ1, γ2.
For example, the substrates S1 and S2 may have been bonded along the interface I2, whilst the interface I1 is an interface formed during the epitaxy of a material on a support, the material and the support together forming the substrate S1.
If the energy γ2 is lower than the energy γ1, when a blade B is inserted between the two substrates S1, S2, the separation will preferably take place along the interface I2 having the lowest rupture energy.
However, it is not necessarily along this interface that it is desired to carry out the separation.
Indeed, it may be preferred to carry out the separation along an interface having a higher rupture energy.
However, the method of inserting the blade does not make it possible to influence the interface along which the separation is initiated.
It is, furthermore, known to use a phenomenon referred to as “stress corrosion” to accelerate the separation of the substrates.
Stress corrosion consists in combining the parting force of the blade with the application of a fluid to the separation interface.
Stress corrosion is particularly benefited from when at least one of the substrates is made of silicon and when the interface comprises silicon oxide, whether it is a native oxide or an oxide formed intentionally, for example, in order to form a bonding layer or an insulating layer.
This is because such an interface contains siloxane (Si—O—Si) bonds that are broken by water molecules provided by the fluid.
The rupture energy of the interface is, thus, significantly reduced.
Chapter 14, entitled “Debonding of Wafer-Bonded Interfaces for Handling and Transfer Applications,” by J. Bagdahn and M. Petzold, in the book “Wafer Bonding: Applications and Technology,” edited by M. Alexe and U. Gösele, Springer, 2004, describes this stress corrosion phenomenon and proposes various applications.
In particular, this document especially envisages a transfer process for producing microelectromechanical systems (MEMS), in which a substrate of silicon-on-insulator (SOI) type is bonded to a silicon substrate having a cavity, then the SOI is debonded along the interface between the thin layer of silicon and the buried oxide layer.
Stress corrosion makes it possible to reduce the rupture energy of the interface even if, due to a heat treatment for strengthening the interface, it initially has a higher energy than that of the other interface.
However, insofar as the interface between the thin layer of silicon, and the buried oxide layer of the SOI has a rupture energy higher than that of the interface between the SOI and the silicon substrate, the authors do not explain how they succeed in starting the debonding along this first interface.
Consequently, the means for the concrete implementation of this process are not defined.
One objective of the invention is to be able to separate a structure comprising several interfaces by taking advantage of stress corrosion, but by controlling, among these various interfaces, the interface along which the separation must take place.
One objective of the invention is, thus, to solve the problems described above, and to propose a process for fabrication of a structure by assembling at least two substrates, the structure comprising at least two separation interfaces, making it possible to select the interface along which a separation, assisted by stress corrosion, will take place.